1. Field of the Invention
This invention relates to a curable resin composition. More particularly, this invention relates to a curable resin composition containing a microparticulate form of silicone rubber. The composition exhibits an improved fluidity during molding, and converts to a cured material which itelf has an excellent flexibility, a low thermal expansion coefficient, a low mold shrinkage ratio and exhibits excellent adhesion between said resin and the silicone rubber particles.
2. Description of the Prior Art
Curable resin compositions have excellent electrical properties, for example, dielectric properties, volume resistivity, and dielectric breakdown strength, in addition to excellent mechanical properties, for example, flexural strength, compression strength, and impact strength. These properties make the compositions particularly desirable for use as insulating materials for various types of electric and electronic components. These materials are fabricated using methods such as transfer molding, injection molding, potting, casting, powder coating, immersion coating and dipping.
Cured resins prepared using the aforementioned compositions are generally rigid and when used, for example, to seal an electric or electronic component, large mechanical stresses are imparted to the interal elements of the component. As a consequence, the element may not function properly or failure may occur in part of the element. One cause of these undesirable features is the difference in thermal expansion coefficient and post-molding shrinkage ratio between the elements of electric and electronic components and curable resins. The elements of electric and electronic components have very low thermal expansion coefficients and shrinkage ratios while the resins have large values for these properties.
The large differences in thermal expansion and shrinkage ratios are why the elements of electric and electronic components as well as other constituent materials are subjected to excessively large internal stresses during the heating required to seal the component, postcure the resin or the thermal cycling to which the component is subjected during testing and use. Furthermore, these differences in thermal expansion coefficients and shrinkage ratios cause cracks in the resin itself and also cause the appearance of gaps between the electric or electronic component and the resin. The infiltration of water and other undesirable materials into these gaps contributes to deterioration of the elements.
Previous attempts to improve the properties of curable resins have not had as their objective a reduction in the thermal expansion coefficient or post-molding shrinkage rato of the curable resins. For example, Japanese Patent Publication Number 52-36534 [36,534/77] relates to an improvement in the lubricating properties of the surfaces of resin moldings achieved by the addition of an organopolysilsesquixone powder to phenolic resins. Japanese Patent Application Laid Open [Kokai] Number 52-14643 [14,643/77] relates to an improvement in the abrasion resistance to metals which is obtained by filling synthetic resin with the microparticles obtained from a cured material based on a organopolysiloxane and an inorganic filler. The thermal expansion coefficient, post-molding shrinkage ratio, and flexural modulus are unsatisfactory in both of the aforementioned resin compositions.
In Japanese Laid Open Patent Application Number 58-219218 [219,218/83] the present inventors propose a solution to the foregoing problem by adding to the curable resin a cured material containing at least 10 weight % of a straight-chain siloxane fraction. The material is reduced to a microparticulate from following curing. However, the problem with this approach that it is not always easy to reduce an elastomeric cured material to the microparticulate form.
The present inventors in Japanese Laid Open Patent Application Number 59-96122 [96,122/84] propose using the spherical cured material obtained by spraying a curable elastomer composition into a current of heated air. This is a quite excellent method, but suffers from the problem of high costs due to the equipment necessary to produce the spherical cured material.
In view of the aforementioned problems associated with prior art methods, an objective of the present invention is to provide a curable resin composition which exhibits an excellent fluidity during molding, which will neither stain the metal mold nor exude onto the surface of the cured material, has an excellent mold-releasability, and which converts to a cured material having excellent flexibility, a low thermal expansion coefficient, and a low mold shrinkage ratio and in which adhesion between the resin and spherical silicone rubber microparticles is excellent.